Presses for extracting fluid from fluid-containing matter so as to separate the fluid from the remaining matter, particularly for dejuicing of agricultural products, primarily fruit, are known. Such presses include a rotatably supported press container, and a flexible membrane actuated by pressure means subdividing the container so as to define a pressure chamber and a press space therein, and wherein the pressure chamber is provided with an inlet conduit and an outlet conduit for a pressure medium, and the press space is provided with a drainage mechanism.
Devices of this kind are known under the designation "Roto-Pressmatic" manufactured by the firm Bucher. In such presses the drainage mechanism consists of several drainage members extending along the wall of the container within the press space parallel to the axis of the container. Each drainage member has a roof-like shape extending into the press space, and its wing surfaces have perforations, through which the juice is drained off.
So-called open systems are also known, in which the container has perforations in the wall within the region of the press space, so that the juice streams first into a receiving vat disposed below the container.
It has been shown that the pressing of grape pulp by such systems requires a processing time of relatively long duration, which is partly due to an unsatisfactory dejuicing using known systems or devices.
No significant improvement of the output of the press of this type has been obtained by means of various tests using, for example, a larger number of throughgoing openings in the drainage members, or in the wall of open systems in the press space, or increasing the number of drainage members, or increasing their constructional size.
Neither has it been possible to increase the output by covering the drainage mechanism with a textile filter cloth. Experiments in which the pressure-applying process was enhanced by creating a vacuum in the press space, although resulting in a respectable increase of the output, have nevertheless reduced the profitability of the pressing process, due to the resulting higher operating costs.
All of the above-mentioned application possibilities did not contribute sufficiently to the known pressing process, in spite of the considerable additional expense entailed.
These tests have led to the assumption that increased amounts of accumulated juice arise between thickened layers of the mash during and after the filling of the container. Juice bubbles arising in this manner cannot be extracted during the pressing process, as the mash surrounding these bubbles is compressed even further.
This state of affairs has led to the consideration that in presses having a closed container system, namely when using drainage members, pre-dejuicing should only be accomplished after filling of the press has been completed. In such a case the solid ingredients of the mash would settle near the top, so that such large juice bubbles could not be formed in the mash. This state of affairs could be obtained without any trouble, but during the subsequent opening of the drainage mechanism the solid ingredients of the mash were attracted by the suction of the liquid, and in fact coated the liquid in such a manner that the drainage was worsened, and juice bubbles arose anew in the mash.